Apparatus for compensating the operating time of electrical devices



Au 12, 1941. KELLER 2,252,321

APPARATUS FOR COMPENSATING THE OPERATING TIME OF ELECTRICAL DEVICES Filed Nov. 8, 1938 2 Sheets-Shegt l Curren-f-Mi/am g- 12, 1941 E. L. KELLER 2252,1321

APPARATUS FOR COMPENSATING THE OPERATING TIME OF ELECTRICAL DEVICES Filed Nov. 8, 1938 A 2 Sheets-Sheet 2 .032 .048 .054- T/me Se c.

INVENTOR Edward L. Ae/ler- T/me Sec. 4 M

Patented Aug. 12, 1941 UNITED STATES PATENT OFFICE APPARATUS FOR COMPENSATING THE OP- ERATING TIME OF ELECTRICAL DEVICES 2 Claims.

This invention relates to the compensation. of the operating time of electrical devices and will be disclosed herein as applied to an apparatus for indicating very small fractions of time, via, of the order of one-sixtieth of a second.

Timing devices capable of measuring short intervals of time are useful in many applications, one of which is the setting of protective relays widely used in electrical distribution circuits. Timing devices have been proposed heretofore in which an indicator is coupled to a constant speed shaft for movement therewith during the period which is to be measured. Such devices have not proved sufficiently accurate, however, because they do not compensate for the time required for the current through the actuating magnet usually employed, to build up to the operating value, and for accelerationv of the moving parts.

I have invented a novel system of compensating for the operating time of electrical devices. A typical embodiment is a timing mechanism which overcomes the aforementioned objection and provides a highly accurate and fully compensated timer. Such embodiment comprises a clutch having an electro-magnet for actuating it, to connect a register to a constant speed driving member during the time interval to be measured. I provide means, furthermore, whereby the electro-magnet holds the clutch in actuated position for as long after the end of the period being measured as was required for the clutch to become actuated after the beginning of such period. This embodiment of the invention is illustrated in the accompanying drawings and the following detailed description has reference thereto. In the drawings:

Fig. 1 is a partial plan view showing the dial or face of my timer;

Fig. 2 is a bottom plan view;

Fig. 3 is a view partly in section along the line III-III of Fig. 2;

Fig. 4 is a sectional view taken along the line IVIV of Fig. 3 with parts in elevation;

Fig. 5 is a schematic diagram;

Fig. 6 is a set of curves which will be referred to in explaining the theory of the invention; and

Fig. '7 is a set of curves illustrating the calibration fo the device.

Referring now in detail to the drawings and, for the present to Figs. 1 through 4, I mount a dial 10 on a face plate II. A frame l2 comprising plates l3 and [3a connected by spacers I4 is secured to the face plate I l by keepers 15. A shaft l 6 journaled in the frame l2 extends through the dial I0 and is provided with a pointer ll. The pointer ll cooperates with a scale 18.

A synchronous motor I9 is mounted on the frame l2 by means of a bracket 20. A gear 2| on the shaft of the motor I!) meshes with a gear 22 which is loose on the shaft IS.

A sleeve 23 splined on the shaft l6 has a clutch disk 24 adapted to engage a face of the gear 22 and a bearing disk 25. A compression spring 26 disposed between the disk 25 and a collar 21 secured to the shaft normally urges the clutch disk 24 into engagement with the gear 22.

A lever 28 is pivoted on a post 29 and has a bifurcated end 30 cooperating with the bearing disk 25. The other end of the lever is provided with an armature 3| adapted to be actuated by an electro-magnet 32. A spring 33 normally urges the lever 28 in a clockwise direction as viewed in Fig. 3. The tension of the spring 33 may be adjusted by shifting a lever 34 to which itvis connected. The lever 34 is pivoted on a post 35 and may be adjusted by a screw 36 threaded through the lever and engaging the plate l3a. The adjustment of the lever 34 is such that the spring 33 overcomes the spring 26 and normally holds the clutch disk 24 out of engagement with the gear 22. When the armature 3| is attracted by the electro-magnet 32, the lever 28 is moved away from the bearing disk 25 thus permitting the spring 26 to force the clutch disk 24 into driving engagement with the gear 22. A screw 31 is threaded through a bracket 38 secured to the frame 12 for adjusting the retracted position of the armature 3|.

The shaft 16 through a train of reducing gearing indicated generally at 39 drives slow speed pointers 40 and 4| cooperating respectively with scales 42 and 43 on the dial l3.

Referring now more particularly to Fig. 5, it will be observed that the electro-magnet 32 is adapted to be connected, by the bridging of terminals 44 and 45, in series with a resistor 46 and a rectifier 41. A choke including an inductance coil 48 and condensers 49 is connected across the rectifier. Alternating current is delivered to the rectifier by connecting terminals 50 to a suitable source. The synchronous motor I9 is permanently connected across the terminals 50 and runs so long as the latter are connected to a current source of appropriate voltage.

With the motor is operating at constant speed, bridging of the terminals 44 and 45 causes current to flow through the circuit including the magnet 32 and resistor 46. This current builds up somewhat in the manner indicated by curve 5! in Fig. 6. At a certain time after the bridging of the terminals 44 and 45, indicated by T1 in Fig. 6, the current reaches a value indicated by the ordinate 52 at which the magnet attracts its armature, thereby connecting the shaft 16 to the gear 22. A slight additional time, T2, is required for the clutch disk 24, shaft [6 and pointer H to accelerate to the speed of the gear 22. The pointers ll, and 4| travel continuously until the end of the time to be measured. The terminal 44 is then connected to a terminal 53 whereby to short-circuit the magnet 32. On shunting of the magnet 32, the current therethrough decays in accordance with curve 54 in Fig. 6 and after a time, T3, reaches a value indicated by the ordinate 55 at which the spring 33 overcomes the pull of the magnet and retracts the lever 28. This frees the gear 22 from the shaft IE and the friction of the parts driven by the shaft causes the latter to be stopped within a very small time interval. It will be apparent that perfect compensation will require that the sum of T1 and T2 be made equal to T3 plus the small time interval required for decelerating the moving parts. The manner in which this is done will be explained by referring to Fig. '7.

The physical constants of the various elements of the circuit shown in Fig. 5, such as resistance, inductance and capacity are determined, unless known, and the curve showing the growth of current in the circuit including the resistor 46 and the magnet 32 is plotted by substituting appropriate values in the well-known equation. The result is curve 56 shown in Fig. 7. Similarly, the curve showing the decay of current in the short circuited magnet 32 is plotted as shown at 51. As is well known, the rate of growth or decay of current in any circuit depends on the ratio of the inductance of the circuit to the resistance thereof. The minimum current in the magnet 32 which is sufiicient to actuate the lever 28 is then determined. This may easily be done by gradually increasing the current through the magnet by suitable control means and observing the value of the current at which the armature 3i pulls in. In a particular case, this current amounted to about 24 milliamperes indicated by the ordinate 58. The next step is to adjust the device so that the current through the magnet on shunting will decrease to a value at which the spring 33 restores the armature, in the same time, T4, which is required for the current to build up in the circuit including the magnet and resistor 46 in series, to the value necessary to actuate the lever 28.

This adjustment may be effected by turning the screws 36 and 37. The former controls the tension of the spring 33 and thus varies both the current necessary to pull the armature in and that at which the armature is restored. The adjustment of the screw 3! controls only the current necessary to pull the armature in. By suitably adjusting the screws 36 and 31, a condition is finally reached at which the current through the magnet 32 decreases along curve 51 to the drop out value indicated by ordinate 59 after shunting of the magnet, in substantially the same time that is required for the current to build up on curve 55 to the pull in value 58. Exact compensation is obtained when the ordinates 58 and 59 have the same abscissa. The close approach to exact compensation which may be obtained by the invention is illustrated by the slight distance between the ordinates 58 and 59 which is almost infinitesimal and for all practical purposes quite negligible. It will be apparent that for a given adjustment of the screw 36, the time T3 must be no greater than the sum of T1 and T2. If T3 is less than the sum of T1 and T2, the screw 31 may be adjusted to equalize T2 with such sum. If the sum of T1 and T2 is less than T2, no amount of adjustment of screw 31 will effect equalization. In other words, screw 31 may be used to increase the sum of T1 and T2 but cannot effect a decrease in T3.

The scale [8 may be calibrated in any desired manner. In the embodiment shown, I have calibrated the scale in cycles and so designed the drive for the pointer ll that when the terminals 50 are connected to a source of sixty cycle alternating current, the pointer will make a complete revolution in .1 second or in six cycles. It is obvious, however, that the calibration of the scale and the drive for the pointer may be so arranged as to meet the requirements of any particular application of the invention, without departing from the principle thereof.

The voltage of the source to which the terminals 50 are connected should be fairly constant although slight variations do not introduce material errors. With the circuit constants shown on the drawings, a voltage of volts across the terminals 56 gives exceedingly accurate results. On checking the accuracy of the device, it was found that only about one reading out of four was as much as .1 cycle or ,6 of a second in error. The inductance L1 of the magnet 32 is the value obtained with the armature retracted, and L2 the value with the armature attracted.

It will be apparent from the foregoing description that the invention provides a system of compensatin for the operating time of electrical devices whereby a highly accurate timing mechanism may be constructed which is not subject to the errors inherent in devices of this sort which have been known previously. I provide nearly exact compensation for the time required for the current through the clutch actuating magnet to build up to the value at which the clutch is actually operated and for the moving parts to be accelerated. Thus even though the indicator does not start moving at precisely the beginning of the time interval to be measured, this is immaterial since the indicator continues to move after the end of the period, for a time precisely equal to the time lost at the beginning of the period.

The manner of using the invention will be obvious to those skilled in the art. In measuring the operating time of a relay, for example, the terminals 44 and 45 may be bridged simultaneously with the energization of the relay. Similarly, the operation of the relay may be used to cause the bridging of terminals 44 and 53. Because of the resistor 45, it is permissible to bridge all three terminals simultaneously.

While the invention has been disclosed as applied to a timing mechanism, the principle thereof may be employed wherever it is desired to exert a timed control, regardless of whether any indication of the length of the time interval is required. One such use isthe closure of a circuit interrupter for a predetermined short time interval. Other applications will doubtless occur to those skilled in the art.

Although I have illustrated and described herein but a preferred embodiment of the invention, it will be understood that changes therein may be made without departing from the spirit thereof or the scope of the appended claims.

I claim:

1. A system for compensating an electromagnetic timer including a coil, an armature, rotating parts controlled thereby and a restoring spring, to eliminate the error introduced by the time required for the current in the coil to build up to the operating value, and also the additional time required for operation of the armature and acceleration of the rotating parts, said system comprising an energizing circuit in which said coil is adapted to be connected, a resistor in said circuit which, with the resistance and inductance of the coil provides a ratio of overall circuit inductance to overall circuit resistance such that the time required for the current to build up in said coil to the value necessary to actuate the armature is less than the time required for the current in said coil to decrease on de-energization of the coil, to a value at which the armature is retracted, by an amount substantially equal to the difierence between the accelerating and decelerating times of the said moving parts, means for deenergizing said coil, means for adjusting the armature to vary the value of the current in said circuit at which the armature operates, and means for adjusting the restoring spring to vary both the value of current in said circuit at which the armature is retracted and the value of current necessary to actuate the armature.

2. A system for compensating an electromagnetic timer including a coil, an armature, rotating parts controlled thereby and a restoring spring, to eliminate the error introduced by the time required for the current in the coil to build up to the operating value, and also the additional time required for operation of the armature and acceleration of the rotating parts, said system comprising an energizing circuit in which said coil is adapted to be connected, a resistor in said circuit which, with the resistance and inductance of the coil provides a ratio of overall circuit inductance to overall circuit resistance such that the time required for the current to build up in said coil to the value necessary to actuate the armature is less than the time required for the current in said coil to decrease on shunting of the coil, to a value at which the armature is retracted, by an amount substantially equal to the diilerence between the accelerating and decelerating times of the said moving parts, means for shunting said coil, means for adjusting the armature to vary the value of the current in said circuit at which the armature operates, and means for adjusting the restoring spring to vary both the value of current in said circuit at which the armature is retracted and the value of current necessary to actuate the armature.

EDWARD L. KELLER. 

